1. Field of the Invention
This invention relates to article theft detection and more particularly it concerns improvements to article theft detection apparatus of the type wherein saturable magnetic target strips mounted on protected articles react to an applied alternating magnetic field in an interrogation zone to produce detectable magnetic fields at harmonics of the applied field.
2. Description of the Prior Art
French Pat. No. 763,681 to Pierre Arthur Picard discloses an article theft detection apparatus of the type to which this invention applies. As described in that patent, articles to be protected from theft are provided with targets in the form of thin strips of material having a high magnetic permeability which is readily and repeatedly driven into and out of saturation in the presence of an alternating magnetic field. An interrogation antenna is provided at an interrogation zone, e.g. at the exit or just past the check-out counter of a store; and means are provided to cause the antenna to generate an alternating magnetic field at a given frequency and at an intensity sufficient to saturate a target strip in the interrogation zone. As a result, the target strip itself produces magnetic fields which alternate at frequencies which are harmonics of the given transmitter frequency. A receiver antenna is also provided at the interrogation zone to receive the magnetic fields produced by the target strips. The receiver antenna is connected to a receiver which is tuned to detect signals at one or more of the harmonic frequencies produced by the target strip; and an alarm is connected to the receiver to be activated when such detection takes place.
Various refinements to the basic devices shown in the French Pat. No. 763,681 are shown and described in U.S. pending patent application Ser. No. 715,568 filed Aug. 18, 1976 and U.S. Pat. Nos. 4,074,249, 4,118,693, 3,820,103, 3,820,104, 3,673,437, 3,737,735 and 3,534,243. The United States pending patent application Ser. No. 715,568 and U.S. Pat. Nos. 4,074,249 and 4,118,693 disclose the use of a transmitter antenna on one side of an interrogation zone and a receiver antenna on the opposite side of the zone, with both the transmitter and receiver antennas connected in the form of plural offset loops. U.S. Pat. Nos. 3,820,103 and 3,820,104 both disclose the use of a transmitter antenna made of rigid metal pipe in the form of a multi-turn loop. U.S. Pat. Nos. 3,534,243, 3,673,437 and 3,737,735 disclose circuits for generating sine wave signals which can be used to energize transmitter antennas. Also, U.S. Pat. No. 4,016,553 shows transformer couplings to multiple loop transmitter and receiver antennas in a different type detection apparatus.